What is the compound interest formula?

For discrete compounding, the formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the time in years. When monthly contributions are included, each contribution is compounded from the time it is made, which this calculator handles via a month-by-month simulation loop for precision.

How does continuous compounding work?

Continuous compounding treats interest as accruing at every instant. The formula is A = Pe^(rt). In practice, the difference between daily compounding and continuous compounding is very small for typical rates and time horizons. This calculator uses Pe^(rt) for the principal and adds monthly contributions using a continuous accrual factor for each month.

How much does compounding frequency matter?

More frequent compounding yields a higher effective annual rate. For example, 6% compounded monthly has an effective annual rate of about 6.17%, versus exactly 6% compounded annually. The gap grows with the rate and time horizon, but for typical savings rates (under 10%) the practical difference between monthly and daily compounding is less than 0.1% per year.

What does the growth multiplier (x) represent?

The growth multiplier shows how many times larger your final balance is compared to your total contributions (principal plus all monthly deposits). A multiplier of 2.5x means you end up with two-and-a-half times what you put in. It is a quick way to see the power of compounding regardless of the dollar amounts involved.

Are taxes and inflation factored in?

No. This calculator shows nominal (before-tax, before-inflation) growth. In a taxable account, investment gains are subject to capital gains or income tax depending on the account type and jurisdiction. To estimate real (inflation-adjusted) growth, run it again with the real rate: (1 + nominal rate) ÷ (1 + inflation) − 1. For 7% growth and 3% inflation that is about 3.88%, close to the quick estimate of 7 − 3 = 4%.

Are monthly contributions added at the start or end of the month?

At the end. Each month the calculator first adds that month's interest, then adds your contribution, so a deposit starts earning the following month. Deposits made at the start of each month earn one extra month of interest: $200 a month at 7% compounded monthly for 20 years grows to $104,185 when paid at the end of each month and $104,793 when paid at the start.

How long does it take to double my money?

Divide 72 by the annual rate for a quick estimate (the rule of 72). At 7%, money doubles in about 72 ÷ 7 = 10.3 years. The exact figure with annual compounding is ln 2 ÷ ln 1.07 = 10.24 years, and more frequent compounding shortens it slightly. Enter your rate and watch the year-by-year table to see the year your balance passes double.

Compound Interest Calculator

Enter principal, rate, time, and compounding frequency to see your future balance, total interest, and year-by-year growth.

Compound interest follows A = P(1 + r/n)^(nt), where P is the starting amount, r the annual rate, n the compounding periods per year and t the number of years. $10,000 at 7% compounded monthly grows to $40,387 in 20 years, compared with $38,697 when interest compounds once a year.

Investment Details

Currency Symbol

Principal

Annual Interest Rate

Time Period (Years)

Compounding Frequency

Optional

Monthly Contribution

Fill in your investment details and click Calculate

Results and year-by-year table will appear here

Final Balance

—

Total Contributions

—

Interest Earned

—

Growth

—

Year-by-Year Breakdown

All years

Year	Start Balance	Contributions	Interest	End Balance

Quick answer

Examples

At 7% a year	$10,000 only	+ $200/month
Annually	$38,697	$140,204
Semi-annually	$39,593	$142,523
Quarterly	$40,064	$143,739
Monthly	$40,387	$144,573
Daily	$40,547	$144,982
Continuously	$40,552	$144,996

Enter your principal, rate, years and monthly contribution, then click Calculate.

Summary

Enter principal, rate, time, and compounding frequency to see your future balance, total interest, and year-by-year growth.

How it works

Enter the starting principal amount — the initial lump sum you are investing or saving.
Enter the annual interest rate as a percentage (for example, 7 for 7%).
Set the number of years you want to project the investment.
Choose how often interest compounds: annually, semi-annually, quarterly, monthly, daily, or continuously.
Optionally enter a monthly contribution to see how regular deposits accelerate growth, then review the final balance, total interest earned, and year-by-year breakdown table.

Use cases

A young investor modeling how $10,000 invested today grows over 30 years at 8% compounded monthly, with $200 added each month.
A parent calculating how much a college savings account will be worth by the time their newborn turns 18, given a fixed annual contribution.
A personal finance student comparing the difference between annual, monthly, and daily compounding on the same principal and rate.
A retiree projecting how long a lump-sum nest egg will grow at a conservative 4% rate before withdrawals begin.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu